It has been known for a long time that maintaining the stoichiometric ratio between air and fuel in a pulverized fuel fired process is an important criterion to minimize emissions such as NOx and CO. For example, a pulverized coal (PC) boiler constitutes a large number of burners. It has been both observed and proved that the stoichiometric ratio between air and fuel has to be maintained on a per burner basis. Therefore, both the fuel flow and the airflow are measured, and either the airflow or the fuel flow is used as the control variable, to keep the ratio between the fuel flow and the airflow for each individual burner within strict limits.
It has been proved that matching the airflow to the fuel flow for each individual burner reduces emissions as well as improves other variables in the operation of a solid fuel fired boiler. However, it has been identified that only matching the airflow and the fuel flow does not provide the minimum emission level for the NOx, CO and other adjacent emissions.
The typical and current air/fuel balancing concept has been described in the scheme of FIG. 1. As illustrated in FIG. 1, typical air/fuel balancing method is based on air flow and coal flow measurements for each individual burner. Please note, that the total amount of air is matched with the total amount of fuel by keeping the O2 concentration in the exhaust gas on a certain level (e.g. 2%). The point of the known optimization methods is to keep the same share of fuel and air on each burner. If one burner carries a higher amount of fuel, a higher amount of air should be distributed to that burner. That is, the percent fuel and the percent air on one burner should be the same. However, now it has been surprisingly observed that occasionally either more or less air than the stoichiometric ratio would suggest, is needed for a certain burner in order to minimize the emissions. The reason for this phenomenon is unknown, but has most likely a connection to the mixing properties of the fuel and air in the flame. Therefore, a need exists in the industry for a method of optimizing air/fuel ratio wherein optimization will be made more efficiently being based on the measurements of the actual process conditions.
The present invention relates further to soot cleaning optimization. Minimizing of emissions such as NOx, decreases also the need for soothing. Cleaning particles (fouling) from surfaces is a routine that is fairly common in the process industry. For example, when running a combustion process it is essential to keep heat exchanger surfaces clean for the sake of efficiency. Many different kinds of soot cleaners (blowers) are used and they are run according to a certain sequence to keep the heat exchange surfaces as clean as possible. The soot cleaning is generally done by blowing steam on the heat transfer surfaces or by using pressurized air or sound waves to remove the particle layer, mainly soot from the heat transfer surfaces. The particles released from the heat transfer surface section that is soot blown are then entrained into the exhaust gas stream.
Running soot cleaners is expensive. Furthermore, cleaning heat exchanger tubes with steam, without any particle layer on their surfaces, is very eroding for the walls of these tubes. Erosion of the heat exchanger tubes is again a very expensive affair. However, high expenses will emerge as well if soot cleaners are not used at all. Therefore, it is of great importance to optimize the soot cleaning process thoroughly.
Typically the need for the soot cleaning is estimated from raised exhaust gas temperatures and possible steam temperature anomalies. Some systems weight the heat transfer tubes and on the basis of the mass of the tubes estimate the amount of the fouling on the tubes. Information obtained by these methods does not necessarily give the precise information about which heat exchanger tubes has the most part of the soot stuck to its surface and which tubes are fairly clean.
Therefore, a need exists in the industry for a method of optimizing soot cleaning whereby the soot cleaning will be made more economically and efficiently being based on the measurements of the actual process conditions.